Ok-
GA-Anything by Carendo, or the Real Air Scout Package or SIA Marchetti, Flight 1 stuff ( Eg Pilatus, A36, Cessna 172).
Commercial- For excellent exterior models, go CLS. For the VC's go PMDG ( 737,747). Level D 767 is amazingly accurate, you've got Dreamfleets 727, the Flight1 Super 80. There are also some big packages, Like Aerosims Classic Jets package, or the Overland Package, which has a lot of aircraft! You might also like the CaptainSim 757, but there isn't a freeware panel for it yet and the systems aren't as complicated as PMDG or LDS, or theres the Captain Sim 727 or 707 if you prefer old jets. There are also a lot of products published by Just Flight, such as the 777 professional.
Commuter- PMDG Beech series is nice ( More models in C package but more liveries in the D package), Flight1 ATR 42-500, or the Eaglesoft buisiness jets are good.
International Airports-FlyTampa's stuff is good, especially things like Vienna LOWW. For the UK, Garry Summons UK2000 scenery is very good value for money, and his latest release, Gatwick Pro is really impressive. Also some of the Aerosoft stuff is good, I really like Frankfurt. Cloud 9 do a nice Amsterdam Schipol and LAX,
GA Airport- Again, Garry Summons UK2000 comes with a lot of stuff, or you might want to look at Aerosoft Lukla package as thats something different.
Some different aircraft, CaptainSim C-130 is amazing if your system can handle it, the Realair Spitfire is a dream, or something from Shockwave.
This is not a list of all the best stuff, This is just a list of the stuff that I mostly own that I think are outstanding. Anyone else can feel free to add anything they want, and I hope this is of some use Alex

, If you want I can add some links?
Cheers
John.